Method of making refractory aluminum nitride coatings



METHOD 3F MAKING REFRACTORY ALUMINUM HITRIDE COATINGS D. R. PENDSE O 7 91 1 3 h C r m Filed Jan. 31, 1967 06004 774 max/um 77/ 294 055 Unitedbtatcs idatent US. Cl. 117-93.1 7 Claims ABSTRACT OF THE DISCLOSURE Amethod of applying an aluminum nitride coating on a substrate whichcomprisesmgw spraying aluminum onto the substrate using a nitrogengamsma whereby a mixed coating of aluminum and aluminum nitride isformed. The coating is then nitrided by heat treatment in nitrogen inthe presence of a catalyst such as lithium fluoride.

This invention relates to refractory nitrides and, more particularly, torefractory nitride coatings.

A number of refractory nitrides, particularly those of aluminum, siliconand titanium, are well known for their high resistance to attack bymolten metals such as aluminum, gallium, cadmium, and zinc. They arehowever expensive materials to produce and it is exceedingly difficultto fabricate equipment from such refractory nitrides by the usualcompaction techniques.

When such refractory nitrides are employed mainly for their resistanceto molten metals they can be used as a thin layer coated upon a suitablerefractory substrate, such as aluminous concrete (a ceramic materialhigh in A1 which is inexpensive and provides the necessary mechanicalstrength. Other substrates such as steel and graphite can also be used.

An object of the present invention is to provide a methed whereby acomparatively inexpensive refractory substrate, such as aluminousconcrete, may be coated withv a thin layer of a refractory nitride.

According to the present invention there is provided a method of formingon a substrate a coating of a refractory nitride which comprisesdepositing the nitride on the substrate from a highly ionized gas(plasma) which is sprayed onto the substrate. The gas forming the plasmashould be one that does not react with the substrate and that permitsthe formation of the nitride when this is present as its dissociatedelements, nitrogen being preferred.

The nitride may be fed into the plasma in finely divided or rod form andbe carried by the plasma to the substrate.

Preferably however the nitride is formed in the plasma by feeding into aplasma of nitrogen a metal or other elewhich will decompose in theplasma to yield the desiredmetal or other element which then reacts withthe nitrogen. When it is desired to form titanium nitride, anapproprinte compound which will decompose in the plasma is titaniumhydrideaother metals which can be used are aluminum and silicon.

When the nitride, metal or other element in elemental form, ordecomposible compound is fed into the plasma as a finely divided powderit is most suitably a powder having a particle size of about 200 BSmesh, and may be suspended in an inert carrier gas, preferably nitrogen.

The plasma can be formed with a plasma generator such as a conventionalplasma torch, in which an arc is struck betwcentwo electrodes and thegas to be ionized 3,593,787 Patented Mar. 31,1970

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is blown through the arc. It is then preferred to feed the material tobe deposited into the gas after the gas has been ionized by the arc.Alternatively, as when the nitride to be coated is aluminum nitride, theplasma may be formed by ionization of nitrogen between two electrodesone of which is a sacrificial electrode, the sacrificial elec trodebeing the element, such as aluminum, which is to form the nitride.

The invention will now be described with reference to the accompanyingdrawing which is a sectional side elevation of a plasma torch.

The torch comprises a forwardly tapered generally cylindrical casing 1having an axial bore 2 which is open at its forward end and communicatesat its rear end with an axial counter bore wherein there is an axiallyextending electrode 3 provided with internal cooling means. The wall ofthe casing surrounding the electrode is hollow 4 to provide a passagefor cooling liquid. A radial extending inlet passage 5 communicates withthe bore in the forward portion of the casing.

In use of the torch, the electrode and the casing are connected torespective terminals of a source of electric current, and a stream ofargon is fed forwardly through the counter bore until a plasma begins toform. The argon is then gradually replaced by nitrogen gas, and powderedmetal is fed into the nitrogen plasma through the inlet passage.

EXAMPLE I In one experiment, the following conditions were used: arevoltage, volts; arc current, 200 amps; feed rate, 4 grams per minute;powder size, 180-250 mesh; powder, aluminum. The plasma laden withaluminum nitride was sprayed under an atmosphere of nitrogen onto ashallow sign of damage when molten aluminum was evaporated therefrom.

EXAMPLE II In a similar run, aluminum powder was sprayed on a substrateof steel, which had been sandblasted, at an arcurrent of am 5. Theresultant coating was aluminum with some aluminum nitride. The coatingcan be nitrided further by heat treatment innitrogen in the presence oflithium fluoride catalyst.

EXAMPLE 111 Ina similar run, a composition consisting of 85% aluminumnitride and 15% aluminum was sprayed on a polished graphite substrateusing an arc voltage of 75 volts and are current of 300 amps. A goodadherent coating was obtained.

Various changes and modifications of the invention can be made and, tothe extent that such variations incorporate the spirit of thisinvention, they are intended to be included within the scope of theappended claims.

What is claimed is:

1. The method of forming a coating of aluminum nitride on a substratewhich comprises feeding aluminum to a plasma of nitrogen, depositing amixture of aluminum and aluminum nitride as a coating on said substrate,and nitriding said resultant coating by heat treatment in nitrogen inthe presence of a catalyst.

2. The method according to claim 1 in which said catalyst is lithiumfluoride.

3. The method according to claim 1 in which said suhstrate is selectedfrom the group consisting of steel, graphite and an alumina ceramic.

4. The method according to claim 1 in which said coating is about 0.5mm. thick.

- 3,508,787 3 a 4 5. The method according to claim 1 in which said sub-FOREIGN PATENTS strate is steel. t 6. The method according to claim 1 inwhich said sub- 3/1961 Great Bmam' OTHER REFERENCES strate is graphite.

7. Th6 method according to 1 in Which said sub- Iflarynowgki, et aL;Thermodynamics of selecte strate is an alumina ceramic. Chemical SystemsPotentially Applicable to Plasma Jet Synthesis, I&EC Fundamentals, vol.1, No. 1, p. 52, February 1962.

References Cited UNITED STATES PATENTS X 10 ALFRED L. LEAVITT, PrimaryExaminer 2,201,049 5/1940 Moare 117-53 2.755,199 7/1956 Rossheim et al.117-129 X W Assistant Examiner 3,041,690 7/1962 'My'tton et a1. 1175. 1X

3,114,826 12/1963 Sullivan. 3,271408 9/1966 Winterbum 117-105'.2 X 15117 129 22 143 35 55 2 3,419,404 12/1968 Mao '10665

